Method of generating extreme ultraviolet radiation

ABSTRACT

A method of generating extreme ultraviolet radiation, wherein the radiant medium is a plasma generated by processing a basic material, and the basic material distribution of the radiant medium consists at least of one halogenide of the metals lithium (Li), indium (In), tin (Sn), antimony (Sb), tellurium (Te), aluminum (Al) and/or a halogen 5 and/or an inert gas, with the exception of halogenides on the basis of lithium (Li) and chlorine (Cl) as well as Fluorine (F).

The invention relates to a method of generating extreme ultravioletradiation, wherein the radiant medium is a plasma generated from a basicmaterial distribution.

Such methods are known. They are used, for example, in lithographicprojection for the manufacture of semiconductors. For future generationsof lithographic projection, an intensive light source for short-waveradiation in the extreme ultraviolet range, hereinafter referred to asEUV, from approximately 5 to 50 nm is required. To be precise, as aresult of the availability of efficient multilayer reflectors, the mostpromising concepts will employ a very narrow wavelength band in the 13.5nm range. Generally speaking, the aim is to obtain an EUV light sourcefor lithographic applications, which has a high, overall, usable EUVoutput in the range from 50 W to 100 which output is available uponentering the illumination optical systems, and is necessary to fulfillthe throughput conditions of the lithographic process.

In order to be able to fulfill these extremely high requirements, a highoverall efficiency of the system is required. The most important factorsin respect of the light source are compactness of the radiant volume anda high conversion efficiency of electric input power to EUV radiation.

Various concepts enabling said objects to be achieved are commonlyknown: synchrotron X-ray sources, laser-produced plasmas, hereinafterreferred to as LPP, and discharge sources.

Synchrotron X-ray sources have several disadvantages, which areunacceptable if said sources are to be integrated in a semiconductormanufacturing process. These drawbacks relate to the fact that they areextremely expensive and to substantial requirements regarding the spaceand/or the position occupied by the source and the associatedsurrounding equipment.

The sources for laser-generating the plasma for the EUV range employhigh-power laser beams which are focused on gaseous, liquid or solidtargets and generate a hot plasma emitting the EUV radiation.

The most important drawback of the currently proposed systems is thestrong formation of impurities in the form of ions, atoms or particles,which are emitted by the plasma region.

This may result in a rapid degradation of the EUV radiation-collectionoptics. This problem is most critical for solid targets, but is seriousalso in the case of liquid or gaseous targets, which are generallyemitted by a specific type of nozzle. The oxygen and xenon targets whichare most commonly used with LPPs lead to the problem that a conversionefficiency from electric input power to usable EUV output power ofapproximately 0.25% is achieved, which is very low.

To achieve the necessary EUV levels, a laser beam output of manykilowatts with a pulse frequency above 1 kHz is necessary. These lasersystems are not available at present and even if they were available, asa result of dramatic development efforts, they would be very expensive.

Finally, discharge sources generate EUV radiation by means of anelectrically driven discharge plasma. Various concepts are currentlyunder discussion, for example capillary discharges, z-pinch dischargesas well as hollow cathode-triggered discharges as disclosed, forexample, in DE 199 22 566.

The main advantages of the discharge sources are their compactness,comparatively low costs as well as a direct conversion of the storedelectric energy, leading to the formation of a hot plasma that generatesEUV.

For most LPP and discharge EUV sources xenon is used as the radiantmedium. In spite of the comparatively high conversion efficiency incomparison with other radiant gases, the absolute conversion efficiencyfor Xe sources is approximately 0.5% at the most. Taking into accountsubstantial EUV light losses inside the light-collection andlight-projection optical systems as well as the EUV intensity necessaryat wafer level, the source must supply approximately 100 W EUV powerfrom a very small radiant volume. Division of the necessary power by theconversion efficiency indicated above results in a necessary input powerfor all Xe sources of at least 20 kW, which power must be supplied by alaser or by an electric discharge. This leads to serious technologicalproblems.

The conditions to be fulfilled by the laser system or the electrodedischarge system can be noticeably eased if radiators that are much moreeffective can be used in the plasma-forming process.

Various authors proposed the use of lithium-metal vapor as ahigh-efficiency EUV radiator. For example Partlo et al. in U.S. Pat.Nos. 6,064,072, 6,051,841, 5,763,930, Silfvast et al. in U.S. Pat. Nos.6,031,241, 5,963,616, 5,499,282 and WO 99/34395. Also tin was proposedas an efficient EUV radiator for laser-generated plasma sources, forexample, by T. Tomie et al. in Second Int. Sematec, a Workshop on EUVLithography, San Francisco, October 2000.

In accordance with the prior art described above, only metal vapors fromlithium or tin were used as the radiant medium. It is know, however,that the evaporation of lithium requires very high temperatures of thedischarge system. The evaporation of tin requires even highertemperatures, which cannot easily be realized in a possible gasdischarge source. The metal vapors will be available not only in theplasma volume but also in at least a part of the inner volume of thesource. If the inner parts of the source, which are in contact with themetal vapor, are not hot enough, condensation of the metal occurs. Thiswill most probably lead to a rapid system error. Even if condensationcan be precluded, serious other problems relating to corrosion of theinner parts of the source caused by the hot metal vapor could occur,which is commonly know particularly in the case of lithium.

Therefore, it is an object of the invention to provide a method of thetype mentioned in the opening paragraph, which enables reliable plasmaproduction for EUV generation using simple technical means and avoidsprior art drawbacks.

In the method of the type mentioned in the opening paragraph this objectis achieved in accordance with the invention in that the basic materialdistribution of the radiant medium comprises at least one halogenide ofthe metals lithium (Li), indium (In), tin (Sn), antimony (Sb), tellurium(Te), aluminum (Al) and/or a halogen and/or an inert gas, with theexception of halogenides on the basis of lithium (Li) and chlorine (Cl)as well as fluorine (F).

The use of said media to generate EUV radiation by means of a plasma hasthe important advantage that a predetermined vapor pressure can begenerated at a temperature that is substantially lower than thetemperature that is required if pure metal vapors are used. This leadsto a substantial reduction of the necessary power. If, however, theplasma must be generated at a specific temperature, then this can becarried put by means of the media in accordance with the invention at asubstantially higher vapor pressure.

In accordance with a further advantageous embodiment of the invention,EUV radiation in the range from approximately 5 nm to approximately 50nm is generated. It is thus ensured that the wavelength necessary forlithography is attained.

Furthermore, the generation of a plasma with an electron temperature ofat least 10 eV corresponding to approximately 116,000 K isadvantageously made possible. As a result, effective radiation in theEUV range is achieved.

In accordance with a further advantageous embodiment, at least an inertgas is added to the basic material distribution.

To further increase the temperature advantage, it is advantageous to addat least a further halogenide as a so termed “evaporator” to the basicmaterial distribution.

It is particularly advantageous if said further halogenide is ametal-based halogenide.

In order to further reduce the risk of condensation and/or corrosion ofthe plasma-generating assembly, it may be advantageous to add at least apure halogen to the basic material distribution in a quantity such thatan oversaturation condition of the halogen is obtained.

To achieve a high optical efficiency of the lithographic illuminationand projection optical system, it is advantageously proposed that theemission volume of the extreme ultraviolet principal radiation is below30 mm³.

Furthermore, in accordance with an embodiment of the invention, theextreme ultraviolet radiation is emitted in a wavelength range from 10to 15 nm.

This is advantageous in particular for newer generation-lithographicprocesses wherein Mo—Si multilayer mirrors are used.

Furthermore, the means used for generating the EUV radiation-emittingplasma volume is a discharge taking place between two electrodes.

In accordance with the invention it is also possible that the means forgenerating the EUV radiation-emitting plasma volume is at least onelaser beam.

It is particularly advantageous if the mean pressure of the metalhalogenide, the iodine or another metal halogenide lies in the rangefrom approximately 1 to 1000 Pa.

The plasma can be advantageously generated if the basic materialdistribution comprises at least a metal halogenide in liquid form, i.e.as droplets or as a jet.

Furthermore, in accordance with another embodiment, it is advantageousif the basic material distribution comprises solid metal halogenideparticles, which are transported in a gas stream.

A large variation range of an adapted application is obtained if thebasic material distribution is at least partly gaseous.

Furthermore, it is advantageously possible for the plasma to begenerated in the pulsed mode; however it may alternatively be generatedin the continuous mode of operation.

It is additionally possible that the plasma is generated by a hollowcathode-triggered discharge.

In accordance with a further embodiment of the method, the plasma isformed by a pinch discharge.

WO 01/99143 A1 discloses the formation of halogenides on the basis oflithium with chlorine and fluorine. However, these halogenides exhibitclearly worse vapor pressures than pure lithium. This is shown, interalia, in FIG. 1.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiment(s) described hereinafter.

In the drawings:

FIG. 1 shows vapor pressures of metallic lithium and lithium halogenidesin dependence upon the temperature;

FIG. 2 shows vapor pressures of metallic tin and tin halogenides plottedversus temperature;

FIG. 3 shows vapor pressures of different EUV-emitting halogenides andpure iodine plotted versus temperature;

FIG. 4 shows examples of the gas phase composition versus vaporpressures in the case of a mixture of lithium bromide and tin iodidephases of equal molar quantity; and

FIG. 5 shows examples of the gas phase composition versus vaporpressures in the case of a mixture of lithium iodide and aluminum iodidephases of equal molar quantity.

Hereinbelow a description is given of various examples, with referenceto FIGS. 1 to 5, wherein inter alia the chemical equilibriums of complexgas phases lead to novel attractive possibilities by means of which theproblems in connection with metallic lithium or metallic tin vapor canbe reduced substantially, if not completely. Novel possibilities of EUVradiators are given which are based on lithium, tin or other compounds,or also on pure elements.

As partly set forth hereinabove, it is an object of the invention toprovide:

-   -   novel methods of using lithium (Li) as a radiator in an EUV        source;    -   novel methods of using tin (Sn) as a radiator in an EUV source;    -   novel methods of using lithium (Li) and tin (Sn) as radiators in        an EUV source at temperatures below those required for a pure        metal compound;    -   novel methods of using lithium (Li) and tin (Sn) as radiators in        an EUV source, leading to a reduced risk of corrosion as        compared to that when use is made of pure metals; and    -   novel methods of using other elements of the fifth period of the        periodic system as efficient EUV radiators.

These objects were achieved, inter alia, by supplying the dischargesource with lithium (Li) and tin (Sn) not in the form of pure metalvapor but in the form of various lithium or different halogenides,whether or not together with other existing metallic halogenides.

First the vapor pressure of individual lithium and tin halogenides isconsidered. As shown in FIGS. 1 and 2, the vapor pressure of thehalogenides of lithium and tin can be much higher than that of puremetals. In the case of lithium shown in FIG. 1, for example lithiumiodide can be used as the radiation medium, which is present in the gasphase as a monomer (LiJ)-dimer (Li₂J₂) equilibrium. An overall pressureof lithium-containing elements of the order of 10 ⁻⁴ to 10⁻³ bar, whichis a typical pressure range for the generation of EUV by means of gasdischarge cells, can be attained at a temperature that is approximatelyup to 90 K lower than that necessary for the evaporation of pure metal.At a specific temperature, the overall pressure of thelithium-containing elements is one order of magnitude higher than thevapor pressure of the pure metal in the corresponding temperature zone.

The halogenides shown in FIG. 1 are Li₂J₂, Li₁J₁, Li₂Br₂ and Li₁Br₁,which are compared with the pure metal Li. As shown in FIG. 1, the dimerof lithium iodide, i.e. Li₂J₂ is most advantageous. The vapor pressurevalues of the halogenides on the basis of lithium and chlorine orfluorine however are clearly worse than the vapor pressure of purelithium.

As regards the use of tin in the case shown in FIG. 2, a vapor pressureof 10⁻⁴ to 10⁻³ bar can be attained using, for example, tin chloride(SnCl₂) or tin bromide (SnBr₂) at a temperature in the range fromapproximately 550 K to 600 K In the case of quadrivalent tinhalogenides, for example SnCl₄, SnBr₄ and SnJ₄, this can even beattained at temperatures below 400 K. This temperature is very muchlower than the temperature necessary for the evaporation of the puremetal. At a specific temperature, approximately 900 K in the case shownin FIG. 2, the vapor pressure of SnCl₂ or SnBr₂ is more than 10 ordersof magnitude higher than that of pure tin metal.

FIG. 2 shows the halogenides tin fluoride (SnF₂), tin chloride (SnCl₂)and (SnC₄), tin bromide (SnBr₂) and (SnBr₄) as well as tin iodide (SnJ₂)and (SnJ₄) versus pure tin metal.

As is also shown in FIG. 3, in addition to the halides or halogenides oflithium or/and tin which are known to be used as EUV radiators, alsoother halogenides can be used as efficient EUV radiators. In particularthe elements indium (In), antimony (Sb) and tellurium (Te) showradiation bands in the EUV range. Also for these elements there arehalogenides with a high vapor pressure, which leads to a simplifiedevaporation of sufficiently large quantities in the discharge volume.The temperatures necessary to evaporate sufficient metal halogenide foran EUV-emitting plasma range between 300 K and 600 K.

In addition to said metal halogenides, also elementary iodine (J) can beused as an EUV radiator. The vapor pressure of iodine is very high evenat room temperature (cf. FIG. 1). As a result, also a pure halogen canbe used as an attractive radiator.

The halogenides shown in FIG. 3 are antimony bromide (SbBr₃), telluriumiodide (Te₂J₂), antimony iodide (SbJ₃), tellurium bromide (TeBr₄),indium bromide (InBr, InBr₃, InBr₆), indium iodide (InJ) and, comparedto pure iodine, in this case even J₂.

In addition to pure metallic halogenides also combinations of metallichalogenides can be used. It has surprisingly been found that this canfurther improve the effective pressure of lithium or tin to a valuewhich exceeds even the pressure that can be attained when use is made ofonly a halogenide or a pure halogenide. This effect can be attributed tothe formation of so-termed “heterocomplexes” in the gas phase.

FIG. 4 shows an example of a mixture of equal molar quantities oflithium bromide and tin iodide, use being made of a known method ofcalculating chemical equilibriums. FIG. 4 shows in detail the complexcomposition of the resultant gas phase versus the two halogenides. Inrespect of the EUV discharge source, the most relevant curves are thosethat relate to lithium or tin-containing chemical compositions.

As is indicated by means of arrows, in comparison with FIG. 1, thetemperature necessary to convert the lithium-containing substances tothe gas phase at a vapor pressure of 10-4 bar has been reduced from 800K to 670 K, which can be attributed to the formation of the gas phase ofthe complex lithium tin iodide (LiSnJ₃). In other words, the effectivepressure of the lithium-containing compositions is improved or increasedby more than two orders of magnitude.

An even more efficient example in respect of the increase of theeffective pressure of lithium-containing compositions is shown in FIG.5. Instead of tin iodide (SnJ₂), aluminum iodide (AlJ₃) is used as aso-termed “evaporator” to build up a high gas phase complex pressure oflithium. As is shown in FIG. 5 by means of the arrows, in comparisonwith FIG. 1, the temperature necessary to attain 10⁻⁴ bar forlithium-containing compositions in the gas phase has now been reducedfrom 800 K to 380 K by the formation of the gas phase of the lithiumaluminum iodide complex (LiAlJ₄). The lithium pressure is improved byseveral orders of magnitude with respect to pure lithium iodide, whichcan be attributed to the formation of the gas phase complex withaluminum.

The invention is not limited to said two examples. Other molar fillingratios of the halogenides are also possible and yield good results. Inaddition, the selection of metal halogenides containing lithium or tinas well as the selection of “evaporating” metal halogenide, such as tinhalogenide or aluminum halogenide, is not limited to the examples of themetal halogenides given hereinabove. The entire range of metallichalogenides and of combinations thereof is possible, including“evaporator” halogenides of, for example, gallium, indium, thalliumetc., to obtain a sufficiently increased pressure of lithium ortin-containing compositions in the plasma volume, which is used togenerate EUV radiation.

It is known that the high temperatures within the EUV-generating plasmamay cause molecules, such as metallic halogenides, to decompose intotheir elementary constituents. After they have left the plasma region,said constituents may recombine to their original metallic halogenideform. This may occur in the gas volume as well as at the walls of thedevice, for example at the electrodes in the case of an electricdischarge. In many cases however the operating pressure is very low. Forexample the average free path of the atoms and molecules may be large ascompared to the dimensions of the source system. As a result, therecombination of the constituents of the original metallic halogenidesmay be incomplete. This may possibly lead to the formation of layers orfilms of metallic constituents near the plasma region, for example atthe electrodes of the electric discharge device.

This problem can be precluded by means of an oversaturation of halogensin the system. The additional halogen causes the probability ofrecombination of the metal and the halogen to be increased, therebyremoving the metallic constituents by the formation of volatile metallichalogenides. In this manner, undesirable soiling layers of metallichalogenide constituents can be precluded. In addition, the effectiveconcentration of the metallic halogenide in the region of the plasma canbe increased.

As regards the concept and the operation of a plasma-based generation ofEUV radiation using the above-mentioned constituents, the followingadvantages can be achieved:

-   -   The temperature necessary to evaporate the radiant constituents        can be noticeably reduced or, equivalent thereto, the pressure        or the density of the radiant constituents can be substantially        increased. This leads to a substantial reduction of the        technical problems associated with the generation and        maintenance of hot metal vapors.    -   The temperature level, which is necessary to preclude        undesirable condensation of metal vapor, can be significantly        reduced. This leads to a source design that is technically        simpler and to a smaller thermal load on the source materials.    -   As a result of said possible lower temperature level, corrosion        problems due to aggressive metal vapors can be avoided. As a        result the failure risk of the source can be substantially        reduced.    -   An increased pressure or an increased density of the radiant        constituents leads to a higher EUV-generation efficiency.

1. A method of generating extreme ultraviolet radiation, wherein theradiant medium is a plasma generated from a basic material distribution,characterized in that the basic material distribution of the radiantmedium comprises at least one halogenide of the metals lithium (Li),indium (In), tin (Sn), antimony (Sb), tellurium (Te), aluminum (Al)and/or a halogen and/or an inert gas, with the exception of halogenideson the basis of lithium (Li) and chlorine (Cl) as well as fluorine (F).2. A method as claimed in claim 1, characterized in that extremeultraviolet radiation is generated in the range from approximately 5 nmto approximately 50 nm.
 3. A method as claimed in claim 1, characterizedin that a plasma with an electron temperature of at least 10 eV isgenerated.
 4. A method as claimed in claim 1, characterized in that atleast an inert gas is added to the basic material distribution.
 5. Amethod as claimed in claim 1, characterized in that at least a furtherhalogenide is added as a so-termed “evaporator” to the basic materialdistribution.
 6. A method as claimed in claim 1, characterized in thatsaid further halogenide is a metal-based halogenide.
 7. A method asclaimed in claim 1, characterized in that at least a pure halogen isadded to the basic material distribution in a quantity such that anoversaturation condition of the halogen is obtained.
 8. A method asclaimed in claim 1, characterized in that the main emission volume ofthe extreme ultraviolet radiation is below 30 mm³.
 9. A method asclaimed in claim 1, characterized in that the extreme ultravioletradiation is emitted in a wavelength range from 10 to 15 nm.
 10. Amethod as claimed in claim 1, characterized in that the means forgenerating the EUV radiation-emitting plasma volume is a dischargetaking place between two electrodes.
 11. A method as claimed in claim 1,characterized in that the means for generating the EUVradiation-emitting plasma volume is at least one laser beam.
 12. Amethod as claimed in claim 1, characterized in that the mean pressure ofthe metal halogenide, the iodine or another metal halogenide lies in therange from approximately 1 to 1000 Pa.
 13. A method as claimed in claim1, characterized in that the basic material distribution comprises atleast a metal halogenide in liquid form, i.e. as droplets or as a jet.14. A method as claimed in claim 1, characterized in that the basicmaterial distribution comprises solid and/or liquid metal halogenideparticles which are transported in a gas stream.
 15. A method as claimedin claim 1, characterized in that the basic material distribution is atleast partly gaseous.
 16. A method as claimed in claim 1, characterizedin that the plasma is generated in the pulsed mode.
 17. A method asclaimed in claim 1, characterized in that the plasma is generated in acontinuous mode of operation.
 18. A method as claimed in claim 1,characterized in that the plasma is generated by a hollowcathode-triggered discharge.
 19. A method as claimed in claim 1,characterized in that the plasma is formed by a pinch discharge.